This invention relates to an electromagnetic (EM) switch for a starter, in which a plunger is moved backwardly by the switch being energized to cause a shift lever coupled to the plunger to pivot to close a circuit switch of an electric motor.
FIG. 3 is a sectional view of a conventional electromagnetic switch in which an exciting coil 1 is wound around a bobbin 2, and a fixed iron core 3 supports the bobbin 2. A case 4 holding the fixed iron core 3 functions as a yoke. A plunger 5 is slidably mounted on the front inner surface of the case 4 in the axial direction toward the core. A returning spring 6 biases plunger 5, and a sleeve bearing 7 is fixed on the front portion of the plunger 5 by caulking. A hook 8 has a rear portion thereof held in a bore of the plunger 5 and is slidable in the axial direction. The hook is formed of, for example, plastic, so that the front portion of the hook links with the top portion of a shift lever 24 to cause the lever to pivot. A compression spring 9 biases the hook 8 in a rearward direction.
A rod 10 projects from the rear end portion of the plunger 5, and is either formed integrally therewith or is fixed thereto. An insulation sleeve 11 has the rod movably inserted therein, and a moveable contact 12 is fixed to the rear end portion of the insulation sleeve 11 by an insulation washer 13 and a fixing ring 14. A compression spring 15 having a first end fixed to the plunger 5 and a second end is fixed to the insulation sleeve 11 to bias insulation sleeve 11 of the contact 12. An insulation cap 16 is fixed to the case 4 by caulking and abuts the rear end of the fixed iron core 3 through a rubber packing. A fixed contact terminal 17 is secured to the cap 16 in a position such that a first end of the terminal corresponds to the movable contact 12, and a second end is a terminal bolt 18 connected to a circuit terminal of the motor. A nut 19 and a switch terminal 20 are fixed to the insulation cap 16 to be connected to the lead wire 1a from the exciting coil 1.
A plunger cover 21 covers the front portion of the plunger 5, and has an inner cylindrical edge 21a fixed to the outer surface of the sleeve bearing 7. A flange 21b is inserted inwardly of a supporting rim 4a, and a projection 21c is inserted in an insertion hole 4b disposed on the case 4. A front bracket 22 of the motor is fixed to the electromagnetic switch by a bolt 23 so that the bracket 22 compresses the flange 21b of the plunger cover 21. As mentioned above, shift lever 24 has a top edge portion coupled to the hook 8, so that the shift lever 24 is pivoted by the sliding movement of the plunger 5, and a lower edge portion of the lever 24 causes an overrunning clutch (not shown) to slide. A rubber shield 25 is inserted in a notch in the bracket 22.
Looking at the plunger cover 21 in more detail, a sectional view of the interior of the plunger cover 21 is shown in FIG. 4(A). FIG. 4(B) is a sectional view along the B--B line of FIG. 4(B) showing the plunger cover. FIG. 4(C) is a sectional view of the exterior of the plunger cover and FIG. 4(D) is a sectional view along the D--D line of FIG. 4(A). The plunger cover includes a core plate 26 which is a thin metal disk plate to which a rubber member is heat-stacked (e.g., thermally fused) therearound to form the flange 21b. Two cylindrical projections 26a are formed at the inner surface of the core plate 26 by pressing. The rubber member is thermally fused to the inside of each cylindrical projection 26a to form the projection 21c. The insertion holes 21d positioned on the core plate 26 each have a bolt 23 inserted therethrough so that when the electromagnetic switch is mounted on the bracket 22, the bolt 23 penetrates through the thin rubber at the insertion holes 21d.
The operation of the above-described conventional EM switch is described hereinbelow with reference to the starter motor. When the exciting coil 1 is actuated, the plunger 5 is attracted to the fixed iron core 3. As a result, the hook 8 is slid backwardly to swing the shift lever 24 forwardly. After the hook 8 is slid backwardly by a predetermined amount, the movable contact 12 closes a pair of the fixed contacts 17 to start the rotation of the motor.
However, the above conventional EM switch has a problem in that the flange 21b of the plunger cover 21 includes the core plate 26 which results in a complicated and expensive structure. Furthermore, since the core plate 26 results in the flange 21b having a large thickness, the height (e.g., in the axial direction of the plunger) of the rim portion 4a of the case 4 must be increased (e.g., by about 3 mm). When the height of the rim portion 4a of the case is increased, the case manufacturing system cannot utilize a cold forging technique, but instead must utilize a machining processing. Accordingly, the manufacturing system becomes expensive.